It is well known that many substances are prone to crystallize in different manners, depending on the conditions under which they are crystallized. Different crystalline structures resulting from crystallization of a particular substance are called polymorphs or pseudopolymorphs. It is also known that, when they are melted and cooled rapidly below their melting point, i.e. melt-congealed, the atoms or molecules forming most substances need some time to arrange themselves in the order most natural for the environment in which they are placed. Accordingly, they remain in unstable amorphous or semiamorphous states or organize into metastable polymorphs.
Metastable polymorphs may be enantiotropic, which is a property of certain substances meaning that they can exist in more than one crystal form (Giron, Thermal Analysis and Calorimetric Method in the Characterization of Polymorphs and Solvates, Thermochimica Acta, 248 (1995) 1-59; Parker, Dictionary of Scientific and Technical Terms, McGraw Hill, Inc., 1984, 541; Hancock et al., Characteristics and Significance of the Amorphous State in Pharmaceutical Systems, J. Pharm. Sci., Vol 86, No. 1, 1997, 1-12). Often, there is a relation between the various crystal forms or habit of an enantiotropic substance such that one form is stable above the transition-point temperature and the other is stable below it. Consequently, the crystal habit is dynamic and reversible depending on ambient conditions.
Metastable polymorphs often transform over time into more stable structures. This natural crystallization process is called “aging”, and occurs over time without human intervention. This natural “aging” process is often lengthy and unpredictable, and therefore is costly and potentially dangerous, particularly in the manufacture of pharmaceuticals. The unpredictability arises since the aging process largely depends on environmental factors. Yu, “Inferring Thermodynamic Stability Relationship of Polymorphs from Melting Data”, J. Pharm. Sci., Vol 84, No. 8, 966-974 (1995).
Nevertheless, stable, crystallized substances are generally required for optimum and reliable bioactivity and bioavailability. If metastable particles, for example, microspheres or pellets, are placed in an aqueous medium before full crystallization occurs, deformation of particle shape or even complete destruction of the particles can occur in a matter of hours.
Furthermore, different polymorphs of a particular substance will have different dissolution rates, resulting in a lack of stability and loss of uniformity between different batches of the same drug. For example, Haleblian et al report differences in dissolution rates between polymorphs of fluprednisolone. Haleblian et al. “Isolation and Characterization of Some Solid Phases of Fluorprednisolonef”, J. Pharm. Sci., Vol. 60, No. 10, 1485-1488 (1971).
For pharmaceutical applications, it is particularly important to achieve stable crystallization, because administration of a therapeutic compound often requires suspension in an aqueous solution suitable for injection. Also, even if the compound is not first suspended in an aqueous medium, when it is administered to the patient it is subjected to biological fluids that are water based. The same is true for pellets and implants that are placed in the body through a surgical or other procedure. To assure the physical integrity of the shaped particles and uniform release of the active agent, it is necessary to assure full crystallization prior to administration.
Some workers have attempted to improve the stability of therapeutic compounds by inducing crystallization. For instance, Matsuda et al. suggest modifying crystalline structures using a temperature controlled dispersion drying method. Matsuda et al. “Physicochemical Characterization of Sprayed-Dried Phenylbutazone Polymorphs”, J. Pharm. Sci., Vol 73, No. 2, 73-179 (1984).
However, because dissolution of a solid is also related to surface erosion, the shape and size of the therapeutic particles must also be considered in addition to solubility. Carstensen, “Pharmaceutical Principles of Solids and Solid Dosage Forms”, Wiley Interscience, 63-65, (1977). Thus, when a pharmaceutical compound is administered as a solid or suspension, the preservation of particular shape and size becomes an important factor for assuring the control and reproducibility of bioavailability and biodynamics.
With this in mind, Kawashima et al. proposed a method of spherical crystallization of Tranilast through the use of two mutually insoluble solvents, and conversion of the resulting polymorphs by means of heat. Rawashima et al., “Characterization of Polymorphs of Tranilast Anhydrate and Tranilast Monohydrate When Crystallized by Two Solvent Change Spherical Crystallization Techniques” in J. Pharm. Sci., Vol 80, No. 5, 472-477 (1981).
It has also been reported that the natural process of aging can be accelerated through heating. Ibrahim et al., “Polymorphism of Phenylbutazone: Properties and Compressional Behavior of Crystals” in J. Pharm. Sci., Vol 66, No. 5, 669-673 (1977); Hancock et al., Characteristics and Significance of the Amorphous State in Pharmaceutical Systems, J. Pharm. Sci., Vol 86, No. 1, 1-12 (1997). In some cases, however, the heat required is such that the integrity or shape of the substance is compromised. In several cases where heat has been used, reproducibility of results, stability, and hence control of crystal size within particles has been difficult or even impossible to achieve.
In addition, in some cases the most stable polymorph of a particular substance is a hydrate, rendering it impossible to reach the desired polymorph by means of heat due to resulting dehydration. Furthermore, heating is rarely appropriate for stable crystallization in the case of mixtures. Thus, the process of heat as a method for obtaining stable polymorphs, though superior to the aging process, has significant limitations.
Other workers have studied the use of solvent vapors to induce crystallization of polymeric species. Such efforts include putative crystallization and change of the mechanical properties of polymeric compounds, as described in U.S. Pat. No. 4,897,307. See also Müller, A. J. et al., “Melting behavior, mechanical properties and fracture of crystallized polycarbonates” in Latinoamericana de Metalurgia y Materiales, 5(2), 130-141 (1985); and Tang, F. et al., “Effect of Solvent Vapor on Optical Properties of Pr/sub 4VOPe in polymethylmethacrylates”, in Journal of Applied Physics, 78(10), 5884-7 (1995).
Tang et al. used organic solvent vapors to transform a polymer matrix, Pr4VOPc dye (Vanadyl Phtalocyanine having 4 propyl substituents) from glassy phase I to crystallized phase II. Müller and Paredes describe the crystallization of polycarbonate polymers in terms of the incorporation of solvents or plasticizers into the amorphous state. To the knowledge of the present inventors, such an approach has not been used to form stable crystals of melt-congealed organic compounds and mixtures.